1. Field of the Invention
The present invention relates to a technique of forming a synthetic-resin seal ring in a specific shape such that abutment joint ends of the seal ring face each other with a predetermined gap. The seal ring is molded such that the abutment joint ends, which provided at one point on a circumference of the seal ring and can be joined to each other, are separated from each other.
2. Description of the Related Art
The synthetic-resin seal ring is applied to various instruments including a hydraulic machine in order to seal a liquid, maintain a pressure, and so on. The synthetic-resin seal ring is attached to an annular groove formed in an outer circumferential surface of a shaft to seal an annular gap between the outer circumferential surface of the shaft and a shaft hole into which the shaft is inserted or an inner circumferential surface of a cylinder.
In the synthetic-resin seal ring, usually an abutment joint portion is provided at one point on a circumference of the seal ring in order to enhance an attaching property. Various shapes such as so-called straight cut, bias cut, and step cut are well known as the shape of the abutment joint portion, and the seal ring is molded such that the abutment joint portion thereof is in an opened state (the abutment joint ends thereof are separated from each other).
While the abutment joint portion is in the opened state, an outer diameter of the seal ring is larger than an inner diameter of the attaching space. Therefore, in order to improve an easiness of attaching the seal ring in the attaching space and to stabilize a posture or position of the seal ring in the attaching space to exert a sufficient sealing function, the seal ring is deformed to a specific shape (reduced-diameter deformation) such that the abutment joint portion is in a closed state (the abutment joint ends are substantially joined to each other) in a natural state.
Various methods are well known as the seal-ring shape forming method (for example, see Japanese Patent Nos. 3894752 and 2729886, Japanese Utility Model Publication No. 2-35088, and Japanese Patent Application Laid-Open Nos. 2002-372153 and 2002-89718). For example, there is well known a method, in which plural products (seal rings) are assembled in a cylindrical jig while diameters of the products are reduced and the whole jig is heated by putting the jig in an electric furnace. However, the following problems are generated in the method.
First it is necessary to evenly heat the whole jig to a certain temperature, and it takes a long time to raise the temperature. Accordingly, when a heating time is shortened, sometimes a temperature difference is generated between regions of the jig, and dimensional accuracy of the shape is varied among the products after the above treatment.
Further, because the method is basically a batch type, the number of members that can be processed in one-time working is restricted, and working efficiency is limited in achieving larger-scale production. Accordingly, in order to increase the number of products that can be processed, it is thought that the jig is enlarged or the number of jigs is increased. However, it is necessary that the electric furnace and the like be enlarged with enlarging jig, and jig put-in work and jig take-out work are increased as the number of jigs is increased.
In cases where the abutment joint portion has the shape in which many joint surfaces are formed between the abutment joint ends like the step cut or composite step cut, because the abutment joint ends are easy to fix by the heating, it is necessary that the process be performed with high accuracy.